Multiple Input Multiple Output (MIMO) technology is technology for transmitting signals using a multiple antenna at both a transmission end and a reception end. MIMO technology has high reliability and provides a high data transmission rate by increasing a channel capacity within a limited frequency resource.
Up to now, in order to effectively receive independent signals transmitted from a plurality of transmission antennas, a reception scheme through reiterative information exchange between a decoder and a detector has been actively studied, and in particular, it is known that a reiterative signal detection scheme using Maximum A Posteriori Probability (MAP) provides an almost optimum reception performance. However, in the MAP-based detection scheme, complexity rapidly increases depending on the number of transmission antennas and the number of data bits per transmission signal. To address this complexity problem, a reiterative reception scheme using SIC and an MMSE detection scheme is proposed.
In a conventional MMSE-SIC detection scheme, to detect a signal xm transmitted from an m-th transmission antenna of M transmission antennas during a process of detecting an i-th repetition signal, an interference signal that would have been transmitted from a different antenna is removed from a reception signal r as in Equation 1:ym(i)=r−Hm{tilde over (x)}m(i)  [Eqn. 1]
where an index i is the frequency of repetition signal detection, ym(i) is a signal vector transmitted via an m-th transmission antenna during a process of detecting an i-th repetition signal, r is a reception signal vector, Hm=[h1 . . . hm−1hm+1 . . . hM] is a matrix obtained by removing an m-th column from an N×M-channel matrix H (M is the number of transmission antennas, and N is the number of reception antennas), and
            x      ~        m          (      i      )        =            [                                    x            _                    1                      (            i            )                          ⁢                                  ⁢        …        ⁢                                  ⁢                              x            _                                m            -            1                                (            i            )                          ⁢                              x            _                                m            +            1                                (            i            )                          ⁢                                  ⁢        …        ⁢                                  ⁢                              x            _                    M                      (            i            )                              ]        T  is an average value vector of (M−1) transmission signals except an m-th transmission signal xm to be actually detected. An average value
            x      _        j          (      i      )        =            ∑              x        ∈        Ω              ⁢                  xPr        ⁡                  [                                    x              j                        =            x                    ]                    ⁢              (                              j            =            1                    ,          …          ⁢                                          ,          M          ,                      Ω            ⁢                          :                                          constellation set) of a signal xj transmitted from an j-th transmission antenna, is obtained using an (i−1)-th Log Likelihood Ratio (LLR) of a transmission signal bit calculated by a channel decoder during a prior (i−1)-th repetition process. Since an LLR(i−1) value of a data bit is 0 when an initial signal is detected (i=1), xj(1)=0.
To detect a signal xm transmitted from an m-th transmission antenna of M transmission antennas as in Equation 1, interference signals, that is, signals transmitted from (M−1) different antennas except the m-th transmission antenna are removed from a reception signal, and then an MMSE filtering coefficient Gm(i) is obtained as in Equations 2a and 2b, so that a signal zm(i) actually transmitted from the m-th antenna is detected.zm(i)=(Gm(i))Hym(i)  [Eqn. 2a]Gm(i)=(HmQm(i)Hm+σx2hmhmH+σ2IN)−1σx2hm  [Eqn. 2b]
where ym(i) is a signal vector transmitted via the m-th transmission antenna during a process of detecting an i-th repetition signal,
      Q    m          (      i      )        =                    σ        x        2            ⁢              I        M              -          diag      ⁡              (                                                                                            x                  _                                1                                  (                  i                  )                                                                    2                    ,          …          ⁢                                          ,                                                                                    x                  _                                                  m                  -                  1                                                  (                  i                  )                                                                    2                    ,                                                                                    x                  _                                                  m                  +                  1                                                  (                  i                  )                                                                    2                    ,          …          ⁢                                          ,                                                                                    x                  _                                M                                  (                  i                  )                                                                    2                          )            is an (M−1)×(M−1) interference signal matrix, hm is an m-th column of H, σx2 is power of a transmission signal, σ2 is power of a white noise, IN is an N×N-unit matrix, and (•)H is a conjugate transpose matrix.
Therefore, according to the conventional MMSE-SIC signal detection scheme, an inverse matrix of an N×N matrix should be calculated every process of detecting an repetition signal in order to detect xm. Generally, since an inverse matrix operation represents complexity of O(N3), assuming that maximum L times of soft information exchange are repeated between a decoder and a detector, an operation amount of O(N3ML) is required in order to detect a total of M transmission signals (i=1, 2, . . . , L).
As described above, the MMSE-SIC based signal detection scheme shows low complexity compared to a conventional MAP based signal detection scheme, but an inverse matrix of a square matrix having a size of the number of reception antennas should be calculated whenever information exchange is reiteratively performed between a decoder and a detector. Therefore, high complexity is required in order to detect a repetition signal.